A fiber laser device is used in various fields such as a laser processing field and a medical field because the fiber laser device has excellent light-condensing performance and high power density, and makes it possible to obtain light with a small beam spot. In such a fiber laser device, an output of emitted light is increased. However, as the power density of light in an optical fiber increases, wavelength conversion of light due to stimulated Raman scattering is likely to occur, and light having an unintended wavelength may be emitted. In this case, light reflected by a workpiece or the like returns to the fiber laser device and is amplified again, amplification of light having a wavelength to be amplified in design is thereby unstable, and an output may be unstable.
In order to suppress stimulated Raman scattering in an optical fiber, the effective cross-sectional area of light propagated through a core is increased. Examples of a method for increasing the effective cross-sectional area include a method of increasing the diameter of the core and a method of reducing a relative refractive index difference of the core with respect to a cladding. When the diameter of the core is increased, a light confinement force of the core is increased, and therefore the optical fiber tends to be a multimode fiber. Therefore, in order to suppress the light confinement force of the core, the relative refractive index difference of the core with respect to the cladding is reduced. However, when the relative refractive index difference of the core with respect to the cladding is reduced, the light propagated through the core is easily affected by macrobending or microbending. For this reason, it is required to adjust the relative refractive index difference of the core with respect to the cladding while appropriately increasing the diameter of the core.
However, even in a case where the core is designed as described above, the size of the effective cross-sectional area of light has a limit when it is tried to propagate light in a single mode. Therefore, it is tried to increase the effective cross-sectional area of light by constituting a fiber laser device using an optical fiber having a core capable of propagating light in a few mode like an optical fiber described in the following Patent Literature 1.    [Patent Literature 1] JP 2016-51859 A
A fiber laser device may have excellent beam quality of emitted light from a viewpoint of light-condensing performance or the like. Therefore, even in a case where the effective cross-sectional area of light is increased by using an optical fiber having a core capable of propagating light in a few mode as described above, it is demanded to suppress excitation and oscillation of light in a mode other than a fundamental mode. Note that the beam quality is indicated by, for example, M2 (M square).